Hitherto, titanium materials such as titanium alloys and pure titanium have been widely used in sporting and recreational goods, medical instruments, various plant components, aerospace instruments and the like because they are typically light and strong compared to iron metal materials such as iron and iron alloys.
Also, because of their high corrosion resistance, titanium materials have been used, for example, in plates in a plate heat exchanger, in a muffler of a motorcycle and the like.
In the production of such products, for example, plates made of a titanium material (titanium plates) are subjected to various processes such as bending and drawing which involve plastic deformation.
In view of the use in such a variety of applications, there has been a need for titanium plates that exhibit good workability in forming processes such as drawing.
What is called “commercially pure titanium” is classified, for example, into JIS Type 1, JIS Type 2, JIS Type 3 and JIS Type 4. In terms of material characteristics, it is known that Type 1 has the lowest strength, and the greater the type number, the higher the strength.
Meanwhile, formability decreases as the type number increases, and performing a process such as drawing using larger type number titanium would be difficult.
To address this issue, Patent Documents 1 and 2 describe that formability can be improved by regulating the contents of components other than titanium in “commercially pure titanium” within a predetermined range. It is difficult, however, to expect sufficiently high strength in titanium products described in these documents.
Patent Document 3 describes that products made of a titanium alloy with a predetermined Fe content exhibit good polishability, while Patent Documents 4 and 5 describe that products made of a titanium alloy with a predetermined content of Zr or the like have good polishability.
Articles made of such a titanium alloy as described in Patent Documents 3 to 5 are believed to exhibit good polishability and high strength because of the fine crystal grains and high hardness they have.
However, when titanium plates are made of such titanium alloys as described in Patent Documents 3 to 5, they are not expected to have good workability because processes such as drawing cannot be easily performed, for example.
The problem therefore is that it has conventionally been difficult to produce a titanium plate having both high strength and good workability.    Patent Document 1: Japanese Patent Application Laid-open No. Sho-63-60247    Patent Document 2: Japanese Patent Application Laid-open No. Hei-9-3573    Patent Document 3: Japanese Patent Application Laid-open No. Hei-7-62466    Patent Document 4: Japanese Patent Application Laid-open No. Sho-62-87932    Patent Document 5: Japanese Patent Application Laid-open No. Sho-63-186843